Books that aren't currently in print occasionally see their used price spike on Amazon. As far as I can tell, the spike is meaningless and doesn't reflect that there are any sales occurring at that price point (so you probably shouldn't hustle over to eBay). You just have to wait for the price to go back down.

One thing about eBay is that it will show you sold listings. It's a checkbox on the bottom of the left-hand column. There are people who apparently set a price for an item and just re-list it. The asking price may or may not have anything to do with what people actually are willing to pay.